<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>canvas-连线动画</title>
  </head>

  <body>
    <canvas id="canvas" width="400" height="400"></canvas>
    <script>
      //起点：10，20 终点：150，200
      const points = [
        [200, 100], //上
        [300, 200], //右
        [100, 200], //左
        [200, 100], //上
        [200, 300], //下
        [100, 200], //左
        [300, 200], //右
        [200, 300]
      ];
      const canvas = document.querySelector("canvas");
      const ctx = canvas.getContext("2d");
      const img = new Image();
      const speed = 10; //速度
      img.onload = function() {
        canvas.width = img.width;
        canvas.height = img.height;

        animate(ctx);
      };

      img.src = "./imgs/demo.png";
      function animate(ctx) {
        ctx.drawImage(img, 0, 0);
        points.forEach(([x, y]) => {
          drawDot(x, y);
        });
        lineMove(points);
        function lineMove(points) {
          if (points.length < 2) {
              
            return;
          }
          const [[x1, y1], [x2, y2]] = points;
          let dx = x2 - x1;
          let dy = y2 - y1;
          if (Math.abs(dx) < 1 && Math.abs(dy) < 1) {
            points = points.slice(1);
            lineMove(points);
            return;
          }
          let x = x1,
            y = y1; //线条绘制过程中的终点
          if (dx === 0) {
            (x = x2), (y += (speed * dy) / Math.abs(dy));
          } else if (dy === 0) {
            x += (speed * dx) / Math.abs(dx);
            y = y2;
          } else if (Math.abs(dx) >= 1) {
            let rate = dy / dx;
            x += (speed * dx) / Math.abs(dx);
            y += (speed * rate * dx) / Math.abs(dy);
          }
          drawLine(x1, y1, x, y);
          points[0] = [x, y];
          window.requestAnimationFrame(function() {
            lineMove(points);
          });
        }

        function drawLine(x1, y1, x2, y2) {
          ctx.save();
          ctx.beginPath(); //不写每次都会重绘上次的线
          ctx.lineCap = "round";
          ctx.lineJoin = "round";
          var grd = ctx.createLinearGradient(x1, y1, x2, y2);

          ctx.moveTo(x1, y1);
          ctx.lineTo(x2, y2);
          ctx.closePath();
          ctx.strokeStyle = "rgba(255,255,255,1)";
          ctx.stroke();
          ctx.restore();
        }

        function drawDot(x1, y1, r) {
          ctx.save();
          ctx.beginPath(); //不写会和线连起来
          ctx.fillStyle = "red";
          //绘制成矩形
          ctx.arc(x1, y1, r ? r : 2, 0, 2 * Math.PI);
          ctx.fill();
          ctx.restore();
        }
      }
    </script>
  </body>
</html>
